The present invention relates to a perpendicular magnetic film. More particularly, the present invention relates to a perpendicular magnetic film which is composed of a spinel thin film containing Fe as the main ingredient and further containing Co and Ni, which has an excellent oxidation resistance, an excellent anticorrosivity, and an appropriate coercive force for preventing a magnetic saturation of a magnetic head which is widely used at present, particularly, a coercive force of less than 3000 Oe, and hence, which is suitable as a high-density magnetic recording medium. The present invention also relates to a process for producing such a perpendicular magnetic film, and a multilayered film for a perpendicular magnetic film which is used as an intermediate in the production of the perpendicular magnetic film.
With recent remarkable tendency for miniaturized and higher reliable information processing apparatuses and systems, magnetic recording media have increasingly been required to have a higher recording density. A perpendicular magnetic film as a magnetic recording medium which respond to such demand has been rapidly developed and put to practical use. Perpendicular magnetic films which are magnetized perpendicularly to the film surface are not only free from demagnetization but also capable of high-density recording.
An alloy film such as a CoCr alloy film has conventionally been proposed as a perpendicular magnetic film. However, it is necessary to coat the surface of a CoCr alloy film with a carbon film having a thickness of about 100 to 200 .ANG. in order to prevent the deterioration of the magnetization characteristics. As a result, the spacing (the distance between a magnetic head and the recording medium) loss is increased by the distance corresponding to the thickness of the carbon film, which is unsuitable for high-density recording. For this reason, a material of a perpendicular magnetic film is strongly required to be an oxide which is stable against oxidation.
As to the coercive force of a perpendicular magnetic film, an appropriate coercive for preventing a magnetic saturation of a magnetic head which is widely used at present, particularly, a coercive force of less than 3000 Oe is required.
The coercive force (Hc) of a magnetic recording medium has a close relationship with the performance of a magnetic head, as is well known. When the coercive force (Hc) of the magnetic recording medium is as high as more than 3000 Oe, the current at which a writing operation is performed becomes so high that the head core of a magnetic head which is widely used at present is magnetically saturated due to an insufficient saturation flux density (Bm). As a result, it is impossible to sufficiently magnetize the magnetic recording medium.
A ferrite head is widely used in a magnetic recording and reading-out machine which corresponds to a magnetic recording medium having a coercive force of not more than 1000 Oe, while a head such as a Sendust head, an amorphous head and a thin-film head, whose head core is made of a material having a high saturation flux density, is used in a magnetic recording and reading-out machine which corresponds to a magnetic recording medium having a coercive force of more than 1000 Oe.
As a perpendicular magnetic film for magnetic recording, an amorphous alloy film such as a CoCr alloy film and a CoPt alloy film, a spinel oxide thin film such as a cobalt ferrite film (Japanese Patent Application Laid-Open (KOKAI) Nos. 51-119999 (1976), 63-47359 (1988), 3-17813 (1991), 3-188604 (1991) and 4-10509 (1992)), a magneto plumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949 (1987)) and the like have conventionally been proposed.
Examples of the perpendicular magnetic recording films and processes for producing the same which have conventionally been disclosed are set forth below, concretely.
(i) A process for producing a cobalt ferrite spinel film, which comprises forming a spinel film by sputtering onto a substrate of a low temperature in an oxidizing air by using an alloy target composed mainly of Co and Fe (Japanese Patent Application Laid-Open (KOKAI) No. 63-47359). In Japanese Patent Application Laid-Open (KOKAI) No. 63-47359, the description of "When a reactive sputtering is carried out in an oxidizing air by using an alloy target composed mainly of Co and Fe, a cobalt.ferrite spinel film represented by Co.sub.x Fe.sub.3-x O.sub.4, (wherein 0.5.ltoreq.x.ltoreq.1.05) is formed on the substrate. In this case, even if the surface temperature is as low as about 200.degree. C., a spinel film having a good crystallizability is obtained." is disclosed.
(ii) A magnetic recording medium comprising a substrate, an under layer of a crystalline film having a spinel crystalline structure, and a magnetic recording layer of a spinel ferrite crystalline film, wherein the lattice constant of the under layer is larger than that of the magnetic recording layer (Japanese Patent Application Laid-Open (KOKAI) No. 3-17813). The description of "The under layer is composed of a material represented by the following general formula: EQU AB.sub.2 O.sub.4
wherein A is a metal element of divalent ions and at least one selected from the group consisting of Mg, Mn, Co, Ni, Cu, Zn, Fe, etc., and B is a metal element of trivalent ions and at least one selected from the group consisting of Cr, In, Rh, Sc, Tl, Fe, etc. The magnetic recording layer 3 is composed of the material represented by the following general formula: EQU AB.sub.x Fe.sub.3-x O.sub.4
wherein A and B are the same as in the above-mentioned general formula, and 0.ltoreq.x&lt;2 ." is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 3-17813.
(iii) An oriented film of oxide crystals represented by the following general formula: EQU A.sub.x B.sub.3-x O.sub.y
wherein A is at least one selected from the group consisting of Mn, Co, Ni, Cu, Mg, Cr, Zn, Li and Ti; B is Fe or Al;, 0.5.ltoreq.x.ltoreq.2.0 and 2.5.ltoreq.y.ltoreq.4, wherein the ratio (I.sub.111 /I.sub.222) of the reflective peak intensity I.sub.111 of the crystal face (111) and the reflective peak intensity I.sub.222 of the crystal face (222) is less than 0.2 when the X-ray diffraction peak is indexed on the assumption that the film has a spinel crystalline structure (Japanese Patent Application Laid-Open (KOKAI) No. 3-188604).
(iv) A process for producing a cobalt ferrite film comprising the steps of forming a multi-layer metal film by laminating at least two of Co layer and Fe layer on a substrate, and heat-treating the obtained multi-layer film in an air containing oxygen (Japanese Patent Application Laid-Open (KOKAI) No. 4-10509).
There are the following description in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509.
"The total thickness of the Co layer and the Fe layer in the multi-layer metal film is not more than 100 .ANG.. This is because if the total thickness of the Co layer and Fe layer exceeds 100 .ANG., it is difficult to produce a cobalt ferrite film having a large Kerr rotation angle."
"Seven kinds of multilayers were formed on a glass substrate 3 (Coning 7059, produced by CONING) by sputtering in Ar while using a single Co target and a single Fe target under the following condition, as shown in FIG. 1.
Sputtering condition:
Total sputtering pressure: 2 mtorr PA2 Sputtering current: 0.2 A PA2 Substrate temperature: room temperature
In each of the multilayers, the thickness ratio of a Co layer 1 and an Fe layer 2 was 1:2, and the total film thickness was constantly 2000 .ANG.. These multilayers obtained were 1 layer of (Co/Fe=660 .ANG./1340 .ANG.) , 2 layers of (Co/Fe=330 .ANG./670 .ANG.), 4 layers of (Co/Fe=165 .ANG./335 .ANG.) , 8 layers of (Co/Fe=82.5 .ANG./167.5 .ANG.), 10 layers of (Co/Fe=66 .ANG./134 .ANG.), 20 layers of (Co/Fe=33 .ANG./67 .ANG.), and 40 layers of (Co/Fe=17 .ANG./33 .ANG.). Each of the multilayers was heat-treated in an electric oven in the air in the heat treatment pattern shown in FIG. 3. More specifically, the film was heated at a high raising rate until 300.degree. C., and then heated at a raising rate of 100.degree. C./hr until 500.degree. C. The film was heated at 500.degree. C. for 2 hours, and was then gradually cooled. Thus, a cobalt ferrite film was formed on the glass substrate."
Namely, the cobalt ferrite films disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 are a multilayer (in-plane magnetic film) composed of a cobalt layer and a ferrite layer and a film (perpendicular magnetic film) composed of a cobalt.ferrite oxide. Each of the multi-layer metal films is heat-treated at a temperature of not lower than 500.degree. C. for not less than 2 hours, thereby obtaining a cobalt.ferrite oxide film.
(v) An amorphous alloy film such as a Gd--Co film and a Tb--Fe film, composed of a rare earth metal and a transition metal (Japanese Patent Application Laid-Open (KOKAI) No. 51-119999) and a magneto-plumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949).
Among the above-described perpendicular magnetic films, the cobalt ferrite films which are typical of spinel oxides are stable against oxidation, because they are oxides, and have a large crystalline magnetic anisotropy. Owing to these magnetic characteristics the cobalt ferrite films are considered to be promising as a perpendicular magnetic recording medium.
As the process for producing a cobalt ferrite (CoFe.sub.2 O.sub.4) film, various methods such as sputtering method, vacuum evaporation method and MO-CVD method are known.
However, although a perpendicular magnetic film which has an excellent oxidation resistance, an excellent anticorrosivity, and an appropriate coercive force for preventing a magnetic saturation of a head which is widely used at present is now in the strongest demand, none of the conventional magnetic thin films sufficiently meet these requirements.
For example, a cobalt ferrite (CoFe.sub.2 O.sub.4) film produced by a sputtering method is disadvantageous in that although the easy magnetization axis of a cobalt ferrite (CoFe.sub.2 O.sub.4) film is (100) axis to the plane (400), the plane (400) is likely to orient at random or the plane (111) is likely to orient in parallel with the substrate, so that it is difficult to produce a perpendicular magnetic film. As examples of a method for obtaining a cobalt ferrite (CoFe.sub.2 O.sub.4) film in which the plane (400) is predominantly oriented in parallel with the substrate, (1) the method described in Proceedings of the 9-th Meeting of Magnetic Society of Japan 29PB-10, (2) the method described in Proceedings of the 13-th Meeting of Magnetic Society of Japan, p 246, and (3) the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 (1992) are known.
The method (1) is a method of depositing Fe and Co ionized in an oxygen plasma on an MgAl.sub.2 O.sub.4 substrate or a silica glass substrate heated to 500.degree. C. Since it is necessary to maintain the substrate temperature at a high temperature such as not lower than 500.degree. C. during film formation, the productivity is poor. In addition, in order to raise the substrate temperature to not lower than 500.degree. C., the substrate itself is required to have a high heat-resistance. However, the heat-resistance of glass plastics or the like, which is widely used as a material of the substrate for a perpendicular magnetic recording medium, is insufficient. In this way, since the material of the substrate is limited, it is not advantageous either industrially or economically.
The method (2) is a plasma-excitation MO-CVD method. Since it is necessary to maintain the substrate temperature at 300.degree. to 400.degree. C. in a vacuum during film formation, the productivity is poor, which is industrially and economically disadvantageous.
The method (3) is a method of annealing a multilayered metal film produced by laminating at least two layers of Co and Fe at a temperature of not lower than 500.degree. C. in an atmosphere containing oxygen. Since a high temperature is necessary, the material of the substrate is limited, as described above, which is disadvantageous industrially and economically.
Accordingly, it is required to eliminate the above-described problems in the related art and to provide a perpendicular magnetic film which is composed of an oxide having a spinel crystal structure, and which has an excellent oxidation resistance, an excellent anticorrosivity and an appropriate coercive force.
Also, it is a technical problem to be solved of the present invention to provide a process for producing a perpendicular magnetic film of an oxide having a spinel crystal structure and showing an excellent oxidation resistance, an excellent anticorrosivity and an appropriate coercive force, at a temperature of lower than 500.degree. C. and as low as possible with industrial and economical advantages.
As a result of the present inventors' studies, it has been found that by annealing a Co- and Ni-containing multilayered film comprising as one unit a magnetite [FeOx.multidot.Fe.sub.2 O.sub.3 (0&lt;x.ltoreq.1)] layer which contains or does not contain Co and/or Ni and an oxide layer which contains Co and/or Ni (provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit), which are formed a substrate, the plane (400) of the multilayered film being predominantly oriented in parallel with the substrate, the thickness of one unit being not more than 300 .ANG., the molar ratio of Co to Fe being 0.005 to 0.32, and the molar ratio of Ni to Co being not less than 0.6, at a temperature of 280.degree. to 450.degree. C., the thus-obtained perpendicular magnetic film an excellent oxidation resistance, an excellent anticorrosivity and an appropriate coercive force, for example, less than 3,000 Oe. On the basis of the finding, the present invention has been attained.